1. Field of the Invention
This invention relates to operational amplifiers and, more specifically, to a dynamic operational amplifier.
2. Description of the Prior Art
Dynamic operational amplifiers utilizing dynamic differential stages have been described by Copeland and Rabaey, "Dynamic Amplifier for MOS Technology", Electronics Letters, Volume 15, Pages 301-303, May 1979, and by Hosticka, "Dynamic CMOS Amplifiers", IEEE Journal of Solid-State Circuits, Volume SC-15, No. 5, 1980. Such dynamic amplifiers utilize dynamic (i.e., non-constant) biasing. The dynamic biasing used causes the amplifier to be biased at a large current in the beginning of the voltage differential cycle, thus causing the amplifier to exhibit a large gain-bandwidth product, thus allowing the output voltage of the amplifier to slew toward the desired output voltage at a very rapid rate. The dynamic biasing continuously reduces the biasing current, thus shifting the amplifier from the operating region of large gain-bandwidth product into the region with large low frequency voltage gain. Thus, the amplifier is able to very accurately slew to the correct output voltage at the end of the differential cycle.
The dynamic CMOS differential stages disclosed by Copeland and Hosticka require a positive supply voltage V.sub.DD as well as a negative supply voltage V.sub.SS. Thus, the dynamic operational amplifiers of the prior art are capable of accurately responding to a positive input voltage as great as (V.sub.DD -V.sub.T), where V.sub.T is the threshold voltage of the differential input transistors. Similarly, prior art dynamic operational amplifiers are capable of accurately responding to a negative input voltage of as little as (V.sub.SS +V.sub.T).
In many applications, it is desired to obtain a wide range of output voltages from a dynamic CMOS operational amplifier in response to a wide range of input voltages. In many instances, the available supply voltages are very limited. For example, in many microprocessor applications, the standard available supply voltage is V.sub.DD =+5 volts, and V.sub.SS =0 volts (ground). Utilizing these voltages in conjunction with the dynamic CMOS operational amplifiers of Hosticka and Copeland, the maximum positive input voltage will be equal to (V.sub.DD -V.sub.T), and the maximum negative input voltage will be (0+V.sub.T). The maximum positive and negative output voltages from these prior art dynamic operational amplifiers will approach V.sub.DD and V.sub.SS (ground). Thus, in many applications the prior art dynamic CMOS operational amplifiers may not be used to provide a wide range of output voltages in response to a wide range of input voltages unless an additional power supply is utilized. The use of additional power supplies is of course expensive, and in many applications, so expensive as to prohibit their use.